Wafer dicing plays a critical role in the fabrication of semiconductor devices, devices which are becoming ever smaller and more complex. The classical methods of dicing are based on the use of a diamond saw for silicon wafers thicker than 100 μm or by laser ablation if they are thinner.
Diamond disk saw technology is limited by its low processing speed (for hard materials). The diamond disk saw produces wide, chipped kerf and low quality edge in general, which in turn degrades device yield and lifetime. The technology is expensive, due to rapid diamond disk degradation, and unpractical owing to the need for water cooling and cleaning. Additionally, the performance is limited when the substrate that is being cut is thin.
Another classical laser processing technology, namely laser ablation, is also limited by its low processing speed and a kerf width which reaches 10-20 μm and is too wide for most applications. Furthermore, laser ablation induces cracks, leaves melted residuals and contaminates the cutting area with debris. A wide area heat affected zone can reduce the lifetime and effectiveness of a semiconductor device.
Together with ablation the diamond disk saw technique can not be used for specialty wafers where there may be other surface features, such as dye-attached films for adhesive stacking. Such additions make the traditional sawing or ablation processes more difficult and vulnerable to debris. In order to improve the quality of separated devices other laser processing based methods and apparatus have been developed.
One of which is a laser processing and laser processing apparatus described in a U.S. Pat. No. 6,992,026, published on 31 Jan. 2006. The said method and apparatus allows cutting a work-piece without producing traces of fusion and cracking perpendicularly extending out of a predetermined cutting line on the surface of the work-piece. The surface of the work-piece is irradiated with a pulsed laser beam according to the predetermined cutting line under conditions sufficient to cause multi-photon absorption, where the beam is aligned to produce a focal spot (or condensed point: a high energy/photon density zone) inside the bulk of the work-piece, consequently forming modified area along the predetermined cleaving line by moving the focal spot in the cleaving plain. After creating the modified area the work-piece can be mechanically separated with a relatively small amount of force.
The said processing method and its variations are currently known in the art as “stealth dicing”. All its variations are based on production of internal perforations by a focused pulsed laser beam at a wavelength for which the wafer is transparent, but which is absorbed by nonlinear processes at the focus, e.g. as in the internally etched decorative blocks of glass. The internal perforation leaves the surface top and bottom pristine. The wafers are usually placed on a plastic adhesive tape that is mechanically stretched causing the perforations to crack. It is claimed that no debris, surface cracking or thermal damage, occurs unlike with prior processes. In addition to specialty and multi-layer wafers, microelectromechanical (MEM) system devices can also be separated this way.
The disadvantages of stealth dicing become apparent as, typically, in order to perform Stealth Dicing a high numerical aperture (NA) lens must be applied, which results in a small depth of focus (DOF) and provides tight focusing conditions. This results in multiple cracks extending to random directions on the surface of cleaving and affects the lifetime of devices produced from of said cleaved wafers. Also, stealth dicing has it's draw backs when processing sapphire. These specific disadvantages are not apparent when wafers and substrates are of thicknesses of up to ˜120-140 μm and only require one pass per separation line to be diced. However, for thicker wafers (usually 4″; 6″ sapphire wafers are >140 μm to 250 μm or more), a number of passes per separation line are required. As a consequence, the material is exposed to laser radiation for prolonged periods of time which has unfavorable influence to final device performance and yield. In addition, multi-pass processing slows down the total processing speed and throughput.
Another method for material processing is disclosed in a US patent application No. US2013126573, published on 23 May 2013. A method is provided for the internal processing of a transparent substrate in preparation for a cleaving step. The substrate is irradiated with a focused laser beam that is comprised of pulses having a pulse energy and pulse duration selected to produce a filament within the substrate. The substrate is translated relative to the laser beam to irradiate the substrate and produce an additional filament at one or more additional locations. The resulting filaments form an array defining an internally scribed path for cleaving said substrate. Laser beam parameters may be varied to adjust the filament length and position, and to optionally introduce V-channels or grooves, rendering bevels to the laser-cleaved edges. Preferably, the laser pulses are delivered in a burst train for lowering the energy threshold for filament formation, increasing the filament length, thermally annealing of the filament modification zone to minimize collateral damage, improving process reproducibility, and increasing the processing speed compared with the use of low repetition rate lasers.
The application of this method results in rough processing applicable only to bare materials and is inconvenient for dicing owing to higher pulse energies required, which leads to unfavorable impact on final semiconductor device performance. In particular, if wafers are diced using this method, resulting light-emitting diodes (LED) are characterized by an increased leakage current, which is in case of high brightness (HB) and ultrahigh brightness (UHB) LEDs strongly reduces performance.
Another US patent application No. US2012234807, published on 20 Sep. 2012, describes a laser scribing method with extended depth affectation into a work-piece. The method is based on focusing of a laser beam in such a way that intentional aberrations are introduced. The longitudinal spherical aberration range is adjusted to be sufficient to extend depth of focus into a work-piece with a limited transverse spherical aberration range. The method also results in rough processing by high energy pulses to obtain vertical damage traces inside the work piece. High pulse energy is necessary due to the fact that a low numerical aperture lens (having a focal length of tens of millimeters) must be used, which results in loose focusing conditions—the focal spot has a very smooth spacial intensity profile, therefore resulting in operation conditions where above damage threshold energy density is achieved in a large area with a relatively small peak value. Due to the increased requirements for pulse intensity (needed for optical breakdown) an increase in pulse energy is required and makes the processing unattractive for HB and UHB LED where LED leakage current and chip wall rough cracking is critical as mentioned above.
Prior art methods impose limitations on substrate thickness, material type and processing quality used for wafer separation. In order to process thicker materials the above mentioned technologies require an increase in laser power or number of laser beam passes per separation line. As a consequence, this has advert effects both to the semiconductor device performance and the yield of production.